Increased susceptibility to infection frequently results from genetic and acquired abnormalities in the immune system. These discrete genetic defects pinpoint the role of the various components of the immune system. At the present time, we are studying genetically determined deficiencies in the serum complement system, acquired defects of immunoglobulin production as are found in patients with acquired agammaglobulinemia, and a variety of T cell defects which compromise the acquired cellular immune system.